Since the introduction of artificial limbs, prosthetic feet have taken on various forms and functions. Many of the foot concepts have been patented. However, only a few of such concepts have been embraced by the limb-fitting profession and in turn accepted by the ultimate consumer--the leg amuptee. Until about 1950, the most popular style of foot in use was an articulated-ankle type that allowed the foot to simulate, on a limited basis, the plantar flexion (ball of the foot contacting the walking surface immediatley after heel contact) and dorsiflexion (the lower leg progressing forwardly to a predetermined angle before the heel lifts off the ground) actions of the normal ankle. There were also a few brands of solid rubber feet utilizing an internal core structure, called a "keel", of wood or metal. The keel furnished an attachment by which the foot could be anchored solidly to the leg, there being no articulated ankle. The heel area was also solid rubber, and this made it difficult for the amputee to walk comfortably, for there would be a jolt upon heel contact with the ground. Consequently, most amputees developed a distinctive compensating gait.
World War II brought about a surge in prosthetic research by the U.S. Government, both directly within service hospitals and through grants to universities and individual firms. One of the results of this research was the development, in the 1950s, of an economically-feasible production foot by the University of California Engineering Department in cooperation with a U.S. naval hospital. This foot was widely accepted worldwide, so much as that the articulated ankle foot was largely rendered obsolete. The University of California foot was called a "SACH" foot, meaning solid-ankle, cushion-heel. It was fabricated of shoe-sole crepe rubber, laminated into layers and glued in sections over a hardwood keel. An important characteristic of this SACH foot was the presence of a soft heel cushion, of sponge rubber, that largely eliminated the above-indicated jolt and permitted the amputees to walk more naturally and with a greater degree of comfort.
Early in the 1960s, a greatly-improved SACH foot was developed and gradually became the most popular prosthetic foot in the U.S. It has a hardwood keel and a soft-foam heel cushion, but instead of being otherwise fabricated of shoe-sole crepe rubber, it is fabricated--around the keel--of a polyurethane foam the density of which is relatively high in comparison to the density of the heel cushion region.
Over the years, attempts have been made to improve the SACH foot still further. For example, some SACH feet were developed having flexible keels formed of metal or a synthetic resin, the keel being surrounded by foam synthetic resin. The purpose of the flexible keels is to store energy during the foot-flat and roll-over portions of the walking movement, followed by releasing of energy from the keel during toe-off. However, at least some of such SACH feet did not achieve commercial acceptance, or had problems relative to keel breakage and/or delamination of the interface between the keel and the synthetic resin foam.